The present invention relates to a method of adhering a colored electroplating layer on zinc-electroplated steel articles. The method utilizes an alternating current power supply which supplies electricity having a frequency of 10-20 Hz, voltage of 60-120 V, and current density of 20-80 Amp/dM.sup.2 ; and an aqueous electroplating solution which contains 150-200 g/l of silicate, 15-20 g/l of sodium hydroxide and 5-50 g/l of cobalt salt. The electroplating solution is kept at a constant temperature of 20.degree. C.-30.degree. C. In this method, two stages are utilized to perform the electroplating and adhere a colored electroplating layer so that the surface of the zinc-electroplated layer can adhere a blue electroplated layer. Thus the aesthetics and the corrosion-resistant capability of the zinc-electroplated layer can be enhanced because of the colored electroplated layer adhered to the surface of the zinc-electroplated layer. In the method mentioned hereinabove, the cobalt salts may be replaced by metal salts such as manganese salts of 5-20 g/l, chromium salts of 5-15 g/l, iron salts of 2-15 g/l, and tin salts of 20-40 g/l so that the adhered color of the surface of the zinc-electroplated layer would be brown, green, white, or grey, respectively.
The main object of the present invention is to apply a colored electroplated layer on the surface of a zinc-electroplated article so as to enhance corrosion-resistance. This is achieved by an alternating current electroplating method utilizing a voltage, a current, and a constant temperature of a limited range. In this method, salts contained in the solution and used in the electroplating process result in a colored electroplating layer being adhered to the surface of the zinc-electroplated layer of the zinc-electroplated steel article and the aesthetics and the corrosion resistance of the zinc-electroplated steel article are thereby enhanced by the colored electroplated layer.
Zinc is an amphoteric metal which is very active and which has the corrosion resistance capability. For this reason many commercially available products, which are made of iron or the like, such as home appliances, automobile body, bridge structure, electric power transmission equipment utilize zinc as corrosion resistant layer. Since the surface coating of the zing layer is soft and susceptible to wearing and corrosion, and the zinc layer becomes zinc oxide after being oxidized, the adherence between the zinc oxide and the iron material is deteriorated. This causes the zinc oxide to peel off the iron material. Furthermore, the surface of conventional zinc-plated steel articles is silver white which lacks aesthetics and limits the field of usage. Conventionally, several methods are employed to improve the color of the surface of the zinc-plated layer and to inhibit the consumption of the zinc layer on the surface; e.g. (1) chemical conversion coating for metals; (2) coating treatment; (3) to produce an interference coloring alloy by adding the elements such as titanium, magnesium or manganese during the process of hot-dip galvanizing; and (4) electrochemical coloring treatment. However, each of the above-mentioned methods has the following drawbacks:
(1) Chemical conversion coating for metals:
This method includes phosphates treatment method, chromates treatment method, and sulphurizing etc. Although the chemical conversion coating can be used for the corrosion resistant treatment for the zinc-plated steel articles, it has drawbacks since the chromates and the sulphites are highly toxic and the phosphate is difficult to dissolve. This causes environmental pollution. Furthermore, the corrosion resistant effect of this method is not ideal for high corrosion areas such as areas close to the sea.
(2) Coating treatment:
There are many kinds of resins which are being used in this method, e.g. PVC resin, acrylic resin, epoxy resin, or the like. However, the coating on the zinc-plated steel articles which have been coated with these resins have the tendency to peel off after being exposed. In particular the adherence between a paint and the surface of a zinc-plated layer is not good if the paint is applied on a steel article which has just been zinc-electroplated.
(3) To produce an interference coloring alloy by adding elements such as titanium, magnesium, or manganese during the process of hot-dip galvanizing:
This method produces alloy products in which the uniformity and the recurrence has to be overcome. Furthermore, the interference coloring alloy produced with this method is relatively thin and does not have a good anti-wearing property.
(4) Electrochemical coloring treatment:
In 1962, a Canadian company named Comince developed APCF (zinc anodized treatment procedure) which is an anodized treatment which utilizes zinc chromate and zinc ammonium phosphate electrolyte, with the voltage raised to 200V, to produce a green coating. In 1971, the U.S. Navy set up standards for zinc and zinc alloy anodized treatment coating, in which there are four kinds of colored coating treatment procedures, i.e., APCF (green coating), SSC (light grey coating), SSCV (dark grey coating), and SSMN (brown coating). These manufacturing procedures use solutions such as chromates, hydrofluoric acid, and permanganates, which are highly toxic and highly corrosive. Thus there is a tendency for causing environmental pollution.
The inventor of the present invention mitigates the problems such as coloring the zinc-plated layer and the thinness of the layer. The inventor conducted research in the technique relating to the zinc-plating of steel articles and the different clay coatings.
The present invention provides mainly a method for providing a colored layer adhered to zinc-plated layers which are zinc layers of pure zinc, hot-dip galvanized zinc, electroplated zinc, die casting zinc, and thermal sprayed zinc. In this method, the zinc-plated layer has enhanced adherence, microhardness, and erosion-resistant properties.